1. Field of the Invention
The present invention relates to a magnetic-resonance imaging apparatus for enhancing the homogeneity of a static magnetic field with a shim coil unit.
2. Description of the Related Art
For the magnetic-resonance imaging, the homogeneity of a static magnetic field constitutes an important element in performance evaluation. A method for correcting error-containing magnetic field components so as to enhance the homogeneity of a static magnetic field is known in the art. Two methods are known: a first method and a second method. The first method is called a stationary passive shimming method according to which a static field magnetic unit, gradient magnetic field coil unit and transmit/receive coil are assembled and a plurality of iron segments are properly arranged at a cylindrical gantry into which a subject to be examined is placed. The second method is called an active shimming method according to which dynamic correction can be achieved by properly supplying an electric current through a plurality of shim coils arranged at a gantry. The first method has been extensively employed from the standpoint of achieving a light-weight apparatus. An inhomogeneous magnetic field component of the static magnetic field contains 1st-order components X, Y, Z, 2nd-order components Z.sup.2, XY, X.sup.2, Y.sup.2, . . . , 3rd-order components Z.sup.3, Z.sup.2 X, Z.sup.2 Y, . . . and a shim coil is provided for every magnetic field components. In practice, use is made of a shim coil set corresponding to 13 channels and 18 channels.
A very high-accuracy homogeneity is required in an imaging method, such as an MRS (MR spectroscopy), MRSI (MR spectroscopic imaging) and EPI (echo-planar imaging). The high-accuracy homogeneity can be achieved by carrying a large current through the shim coil. It has been difficult, however, to obtain a high-level homogeneity because a power source loading problem presents a barrier.